1. Field of the Invention
The present invention relates generally to a W-CDMA (Wide-band Code Division Multiple Access) mobile communication system, and in particular, to a transmitting/receiving apparatus and method for reducing a transmission error rate and thus increasing decoding performance at retransmission.
2. Description of the Related Art
Adverse influences on high-speed, high-quality data service are attributed to a channel environment in a mobile communication system. The radio channel environment varies frequently because of signal power changes caused by white noise and fading, shadowing, the Doppler effect that occurs due to the movement and frequent velocity change of a terminal, and interference from other users and multi-path signals. Therefore, aside from conventional technologies in the second or third generation mobile communication system, an advanced technique is required to support wireless high-speed data packet service. In this context, the 3GPP (3rd Generation Partnership Project) and the 3GPP2 commonly addressed the techniques of AMCS (Adaptive Modulation & Coding Scheme) and HARQ (Hybrid Automatic Repeat Request).
The AMCS adjusts a modulation order and a code rate according to changes in downlink channel condition. The downlink channel quality is usually obtained by measuring the SNR (Signal-to-Noise Ratio) of a received signal at a UE (User Equipment). The UE transmits the channel quality information to a BS (Base Station) on an uplink. Then the BS estimates the downlink channel condition based on the channel quality information and determines an appropriate modulation scheme and code rate according to the estimated downlink channel condition.
QPSK (Quadrature Phase Shift Keying), 8PSK (8-ary PSK), and 16QAM (16-ary Quadrature Amplitude Modulation) and code rates of ½ and ¼ are considered in the current high-speed wireless data packet communication system. In AMCS, a BS applies a high-order modulation (e.g., 16QAM and 64QAM) and a high code rate of ¾ to a UE having good channel quality such as its adjacent UEs, and a low-order modulation (e.g., 8PSK and QPSK) and a low code rate of ½ to a UE having bad channel quality such as a UE at a cell boundary. The AMCS reduces interference signals remarkably and improves system performance, as compared to the conventional method relying on high-speed power control.
HARQ is a retransmission control technique to correct errors in initially transmitted data packets. Schemes for implementing HARQ include chase combining (CC), full incremental redundancy (FIR), and partial incremental redundancy (PIR).
With CC, the entire initial transmission packet including systematic bits and parity bits is retransmitted. A receiver combines the retransmission packet with the initial transmission packet stored in a reception buffer. The resulting increase of the transmission reliability of coded bits input to a decoder brings the performance gain of the overall mobile communication system. An approximate 3-dB performance gain is effected on average since combining of the same two packets is equivalent to repeated coding of the packet.
In FIR, a packet having only parity bits, different from an initial transmission packet, is retransmitted to thereby increase a decoding gain. A decoder decodes data using the new parity bits as well as initially transmitted systematic and parity bits. As a result, decoding performance is improved. It is well known in coding theory that a higher performance gain is yielded at a low code rate than by repeated coding. Therefore, FIR is superior to CC in terms of performance gain.
As compared to FIR, PIR is a retransmission scheme in which a packet having systematic bits and new parity bits is retransmitted. A receiver combines the retransmitted systematic bits with initially transmitted systematic bits for decoding, achieving similar effects to those of CC. PIR is also similar to FIR in that the new parity bits are used for decoding. Since PIR is implemented at a relatively high code rate than FIR, PIR is in the middle of FIR and CC in performance.
A combined use of the independent techniques of increasing adaptability to varying channel condition, AMCS and HARQ can improve system performance significantly.
FIG. 1 is a block diagram of a transmitter in a typical high-speed wireless data packet communication system. Referring to FIG. 1, the transmitter includes a channel encoder 110, a rate matching controller 120, an interleaver 130, a modulator 140, and a controller 150.
Upon input of information bits in transport blocks of size N, the channel encoder 110 encodes the information bits at a code rate R (=n/k, n and k are prime), for example, ½ or ¾. With the code rate R, the channel encoder 110 outputs n coded bits for the input of k information bits. The channel encoder 110 can support a plurality of code rates using a mother code rate of ⅙ or ⅕ through symbol puncturing or symbol repetition. The controller 150 controls the code rate.
The future mobile communication system adopts turbo coding considered a more robust channel coding technique for high-speed reliable transmission of multimedia data. It is known that turbo coding has the nearest Shannon Limit performance in BER (Bit Error Rate) at a low SNR. Turbo coding is also adopted in the 1×EV-DV (Evolution in Data and Voice) standards which are under discussion in the 3GPP and 3GPP2.
The output of the channel encoder 110 being a turbo encoder includes systematic bits and parity bits. The systematic bits are information bits to be transmitted and the parity bits are error correction bits added to the information bits for a receiver to correct errors generated during transmission of the information bits at decoding.
The rate matching controller 120 generally matches the data rate of the coded bits generally by transport channel-multiplexing, or by repetition and puncturing if the number of the coded bits is different from that of bits transmitted in the air. To minimize data loss caused by burst errors, the interleaver 130 interleaves the rate-matched bits. Interleaving distributes damaged bits in a fading environment. Therefore, the interleaving allows adjacent bits to be randomly influenced by fading and thus prevents burst errors, increasing channel encoding performance. The modulator 140 maps the interleaved bits to symbols in a modulation scheme determined by the controller 150.
The controller 150 selects the code rate and the modulation scheme according to the radio downlink channel condition. To selectively use QPSK, 8PSK, 16QAM, and 64QAM according to the radio environment, the controller 150 supports AMCS. Though not shown, a UE spreads the modulated data with a plurality of Walsh codes to identify transport channels and with a PN (Pseudorandom Noise) code to identify a BS.
As stated before, the modulator 140 supports various modulation schemes including QPSK, 8PSK, 16QAM and 64QAM with respect to the interleaved bits. As a modulation order increases, the number of bits in one modulation symbol increases. Particularly in a higher-order modulation scheme greater than 8PSK, one modulation symbol includes three or more bits. In this case, bits mapped to one modulation symbol have different transmission reliabilities according to their positions.
With regard to transmission reliability, two bits of a modulation symbol representing a macro region defined by left/right and up/down have a relatively high reliability in an I (In Phase)-Q (Quadrature Phase) signal constellation. The other bits representing a micro region within the macro region have a relatively low reliability.
FIG. 2 illustrates an exemplary signal constellation in 16QAM. Referring to FIG. 2, one 16QAM modulation symbol contains 4 bits [i1, q1, i2, q2] in a reliability pattern [H, H, L, L] (H denotes high reliability and L denotes low reliability). That is, the two upper bits [i1, q1] have a relatively high reliability and the two lower bits [i2, q2], a relatively low reliability. One 64QAM modulation symbol contains 6 bits [i1, q1, i2, q2, i3, q3] in a reliability pattern [H, H, M, M, L, L] (M denotes medium reliability). Similarly, an 8PSK modulation symbol contains 3 bits. One of them has a lower reliability than the other two bits. Thus, a reliability pattern is [H, H, L].
Considering the above reliability patterns, it is preferable to map coded bits output from the channel encoder 110 to regions having different reliabilities according to their significance levels. As stated before, the coded bits are divided into systematic bits and parity bits having different priority levels. In other words, if errors are generated at different rates in a transport channel according to the reliabilities, a receiver can recover original bits more accurately by decoding when the parity bits have errors than when the systematic bits have errors because the systematic bits are actual information and the parity bits are error correction bits.
In this context, SMP (Symbol Mapping method based on Priority) has been proposed in which systematic bits are mapped to a high reliability region and parity bits are mapped to a low reliability region, so that the error rate of the relatively significant systematic bits can be decreased.
Aside from the different reliabilities of coded bits, each modulation symbol is transmitted with a different error rate on a radio channel in a modulation scheme having a modulation order equal to higher than 16QAM. For example, in the signal constellation for 16QAM, 4 coded bits form one modulation symbol and are mapped to one of 16 signal points. The 16 signal points are classified into three regions according to their error rates. As a modulation symbol is farther along a real or imaginary number axis, it has a lower error rate, which means that the receiver identifies the modulation symbol more easily.
FIG. 3 illustrates graphs showing the error probabilities of the regions in a simulation under an AWGN (Additive White Gaussian Noise) environment. As shown in FIG. 2, the 16 modulation symbols are classified into region 1 having a high error probability, region 2 having a medium error probability, and region 3 having a low error probability. For example, modulation symbols 6, 7, 10 and 11 in region 1 have a relatively high error probability.
In packet data retransmission by HARQ, therefore, retransmission with the same reliability and/or error probability as that of initial transmission does not increase retransmission efficiency. Retransmission of specific bits with a consistently low reliability and/or high error probability deteriorates decoding performance since a channel decoder being a turbo decoder has good decoding performance when the LLRs (Log Likelihood Ratios) of input bits are homogeneous. Therefore, there is a need for exploring a novel retransmission technique that improves transmission performance at retransmission.
Techniques for improving transmission performance at retransmission include SRRC (Shifted Retransmission for Reliability Compensation) and BIR (Bit Inverted Retransmission). In the SSRC, the coded bits of a modulation symbol are shifted by a predetermined number of bits, for example, two bits and thus mapped to different reliability parts at a retransmission from those at their initial transmission. In the BIR, the coded bits are inverted and thus mapped to different error probability parts at a retransmission from those at the initial transmission. Those techniques commonly comprise the LLRs of bits input to a turbo decoder and thus improve decoding performance.
To describe the SRRC in more detail, an M-ary modulation symbol includes log2M bits having different reliabilities. For example, four coded bits form one modulation symbol with the two upper bits mapped to a high reliability and the two lower bits mapped to a low reliability in 16QAM, as illustrated in FIG. 2. Two-bit cyclic shifting of the coded bits of each modulation symbol at a retransmission effects averaging the transmission reliabilities of the coded bits, thereby improving decoding performance.
With regard to the BIR, 16 modulation symbols each having 4 coded bits are classified into region 1 having a relatively high error probability, region 3 having a relatively low error probability, and region 2 having a medium probability in 16QAM, as illustrated in FIG. 2. Inversion of the coded bits of each modulation symbol prior to symbol mapping at a retransmission also effects averaging the error probabilities of the coded bits and thus improves system performance at decoding.
Despite the advantage of improved system performance, however, a simple combined use of the above techniques is not effective in their application to systems. Therefore, the techniques need to be combined effectively so that optimum transmission efficiency can be achieved in a CDMA mobile communication system.